<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="../js/jquery-3.3.1.min.js"></script>
    <style>
        .back {
            position: fixed;
            width: 50px;
            height: 50px;
            background-color: pink;
            right: 30px;
            bottom: 100px;
            display: none;
        }
        
        .container {
            width: 1000px;
            height: 500px;
            background-color: skyblue;
            margin: 400px auto;
        }
    </style>
</head>

<body>
    <div class="back">返回顶部</div>
    <div class="container">

    </div>
    <script>
        $(function() {
            var boxTop = $(".container").offset().top;
            $(window).scroll(function() {
                // console.log(11);
                console.log($(document).scrollTop());
                if ($(document).scrollTop() >= boxTop) {
                    $(".back").fadeIn();
                } else {
                    $(".back").fadeOut();
                }
            });
            $(".back").click(function() {
                //$(document).scrollTop(0);
                $("body,html").stop().animate({
                    scrollTop: 0
                })
            })
        })
    </script>
</body>

</html>